---
title: Database Strategy for Microservices
summary: Architectural decision record for database selection and data management in the FlowMart e-commerce platform
sidebar:
    label: Database Strategy
    order: 4
---

## ADR-004: Database Strategy for Microservices in FlowMart E-commerce Platform

### Status

Approved (2024-08-20)

### Context

Our transition from a monolithic architecture to microservices necessitates a reassessment of our database strategy. The existing monolithic application uses a centralized Oracle database with hundreds of tables spanning multiple business domains. This approach has created several challenges:

1. **Schema Coupling**: Changes to database schemas require careful coordination across teams to avoid breaking dependent services.
2. **Scalability Limitations**: The relational database becomes a bottleneck during high-traffic periods, affecting all application features.
3. **One-Size-Fits-All Approach**: Different data access patterns are all forced into the same relational model, leading to suboptimal performance.
4. **Operational Overhead**: Database administration is complex due to competing requirements from different application components.
5. **Development Bottlenecks**: Teams must coordinate database changes, slowing down development velocity.
6. **Cost Efficiency**: The enterprise database licensing model is expensive and doesn't scale cost-effectively with our varying workloads.

As we decompose our application into microservices, we need a database strategy that promotes service autonomy while ensuring data consistency, performance, and operational efficiency.

### Decision

We will adopt a **polyglot persistence strategy** for our microservices architecture, following the principle of "right database for the right job." Key aspects of this strategy include:

1. **Database-per-Service Pattern**:
   - Each microservice will own and exclusively manage its database
   - No direct database access from other services
   - Services interact via well-defined APIs, not shared databases

2. **Primary Database Technologies**:
   - **PostgreSQL**: Primary relational database for services with complex transactional requirements
   - **MongoDB**: Document database for services with flexible schema requirements and read-heavy workloads
   - **Redis**: In-memory data store for caching, session management, and real-time features
   - **Elasticsearch**: Search engine for product catalog and content search
   - **Apache Cassandra**: Wide-column store for high-volume time-series data and analytics
   - **Amazon DynamoDB**: Managed NoSQL for highly scalable microservices with predictable access patterns

3. **Data Consistency Patterns**:
   - **Saga Pattern**: For coordinating transactions across multiple services
   - **Event Sourcing**: For critical business domains requiring complete audit trails
   - **CQRS**: For services with asymmetric read/write loads
   - **Outbox Pattern**: For reliable event publishing during state changes

4. **Data Migration and Evolution**:
   - Versioned database schemas
   - Backward-compatible schema changes
   - Blue/green deployment for database changes
   - Incremental data migration approach

5. **Data Governance**:
   - Centralized data catalog and lineage tracking
   - Common data models for shared business entities
   - Standard approach to master data management
   - Automated data quality monitoring

6. **Operational Excellence**:
   - Infrastructure as Code (IaC) for all database resources
   - Automated backup and recovery procedures
   - Standardized monitoring and alerting
   - Database reliability engineering practices

### Database Selection Criteria by Domain

| Domain | Database Type | Primary Factors | Secondary Considerations |
|--------|---------------|-----------------|--------------------------|
| Product Catalog | MongoDB + Elasticsearch | Schema flexibility, Search capabilities | High read-to-write ratio, Caching |
| Order Management | PostgreSQL | ACID transactions, Complex queries | Event sourcing for audit trail |
| Customer Profile | PostgreSQL | Data consistency, Relational integrity | Privacy compliance, Scalable reads |
| Inventory | MongoDB | Frequent schema evolution, High write throughput | Eventual consistency model |
| Cart & Checkout | Redis + PostgreSQL | Low latency, High availability | Transaction support for checkout |
| Payment Processing | PostgreSQL | Strong consistency, Transaction support | Compliance requirements, Security |
| Analytics | Cassandra | High write throughput, Time-series data | Analytical query patterns |
| Recommendations | Redis + MongoDB | Low latency reads, Flexible data model | Machine learning feature support |
| User Sessions | Redis | Ultra-low latency, Expiration support | High availability, Cross-region replication |
| Content Management | MongoDB | Flexible schema, Document structure | Full-text search capability |

### Consequences

#### Positive

1. **Service Autonomy**: Teams can independently select, optimize, and evolve their databases without cross-team coordination.

2. **Optimized Performance**: Each service can use a database technology optimized for its specific data access patterns.

3. **Independent Scaling**: Database resources can be scaled according to the specific needs of each service.

4. **Improved Resilience**: Database failures are isolated to specific services rather than affecting the entire system.

5. **Fit-for-Purpose Solutions**: Different data models (relational, document, key-value, etc.) can be applied where most appropriate.

6. **Cost Optimization**: Resources can be allocated based on the specific needs of each service.

7. **Incremental Adoption**: New database technologies can be introduced for new services without migrating legacy data.

#### Negative

1. **Increased Operational Complexity**: Managing multiple database technologies requires broader expertise and more sophisticated tooling.

2. **Data Consistency Challenges**: Maintaining consistency across service boundaries requires careful design and implementation.

3. **Learning Curve**: Development teams need to learn multiple database technologies and data access patterns.

4. **Distributed Transactions**: Business processes spanning multiple services require more complex transaction management.

5. **Increased Infrastructure Costs**: Running and maintaining multiple database systems may increase overall infrastructure costs.

6. **Data Duplication**: Some data may need to be duplicated across services, creating synchronization challenges.

7. **Monitoring and Troubleshooting Complexity**: Different databases require different monitoring approaches and expertise.

### Mitigation Strategies

1. **Platform Database Service**:
   - Create an internal platform team that provides database-as-a-service capabilities
   - Standardize on a limited set of supported database technologies
   - Provide automated provisioning, backup, and monitoring

2. **Data Access Layer Pattern**:
   - Create standardized libraries for common database access patterns
   - Abstract database-specific details behind consistent interfaces
   - Implement retry logic, circuit breakers, and observability

3. **Data Synchronization Framework**:
   - Implement a standardized approach to CDC (Change Data Capture)
   - Create reusable components for the Outbox Pattern
   - Develop standard data synchronization workflows

4. **Database Reliability Engineering**:
   - Establish dedicated database reliability engineers
   - Create runbooks for common database operations
   - Implement chaos engineering practices for database failure testing

5. **Developer Training and Support**:
   - Comprehensive training program for supported database technologies
   - Internal knowledge base and best practice documentation
   - Database design review process for new services

### Implementation Details

#### Phase 1: Foundation (Q3-Q4 2024)

1. Establish database platform team and core services
2. Set up standard PostgreSQL and MongoDB offerings
3. Implement database CI/CD pipelines
4. Develop initial monitoring and alerting
5. Create database provisioning automation

#### Phase 2: Expansion (Q1-Q2 2025)

1. Add Redis and Elasticsearch to supported offerings
2. Implement data synchronization framework
3. Develop CQRS and Event Sourcing patterns
4. Create data governance tooling
5. Expand monitoring capabilities

#### Phase 3: Advanced Capabilities (Q3-Q4 2025)

1. Add Cassandra and specialized databases
2. Implement advanced high availability features
3. Develop cross-region replication capabilities
4. Create advanced analytics integration
5. Implement predictive scaling and optimization

### Considered Alternatives

#### 1. Shared Database Approach

**Pros**: Simplicity, familiar model, transactional integrity, easier reporting  
**Cons**: Tight coupling, scalability limitations, schema coordination challenges

This approach would minimize short-term changes but would recreate many of the current issues in our new architecture.

#### 2. Single Database Technology (PostgreSQL Only)

**Pros**: Operational simplicity, consistent expertise, familiar tooling  
**Cons**: Not optimal for all workloads, potential performance compromises

While PostgreSQL is versatile, it isn't the best choice for all our diverse data requirements.

#### 3. Database-as-a-Service Only (e.g., AWS RDS/DynamoDB)

**Pros**: Reduced operational overhead, managed scaling, built-in high availability  
**Cons**: Vendor lock-in, potential cost concerns, less flexibility

While we'll use managed services where appropriate, we need the flexibility to run certain databases on our own infrastructure.

#### 4. Fully Centralized Data Lake Approach

**Pros**: Centralized analytics, simplified data governance, consistent data model  
**Cons**: Complex real-time synchronization, potential performance issues, development overhead

This approach works well for analytics but isn't suitable for operational data needs.

### References

1. Kleppmann, Martin. "Designing Data-Intensive Applications" (O'Reilly Media)
2. Vernon, Vaughn. "Implementing Domain-Driven Design" (Addison-Wesley)
3. Fowler, Martin. "PolyglotPersistence" [martinfowler.com](https://martinfowler.com/bliki/PolyglotPersistence.html)
4. Richardson, Chris. "Microservices Patterns" (Manning Publications)
5. [Database per Service Pattern](https://microservices.io/patterns/data/database-per-service.html)
6. [Saga Pattern](https://microservices.io/patterns/data/saga.html)

### Decision Record History

| Date | Version | Description | Author |
|------|---------|-------------|--------|
| 2024-07-20 | 0.1 | Initial draft | Robert Kim |
| 2024-08-02 | 0.2 | Added domain-specific database recommendations | Maria Garcia |
| 2024-08-15 | 0.3 | Incorporated feedback from architecture review | David Boyne |
| 2024-08-20 | 1.0 | Approved by Architecture Board | Architecture Board |

## Appendix A: Database Architecture Overview

```mermaid
flowchart TB
    subgraph "Service Domains"
        subgraph "Product Domain"
            pc[Product Catalog]
            ps[Product Search]
            pc --- MongoDB
            ps --- Elasticsearch
        end
        
        subgraph "Order Domain"
            om[Order Management]
            oh[Order History]
            om --- PostgreSQL
            oh --- MongoDB
        end
        
        subgraph "Customer Domain"
            cp[Customer Profile]
            auth[Authentication]
            sess[Sessions]
            cp --- PostgreSQL
            auth --- PostgreSQL
            sess --- Redis
        end
        
        subgraph "Inventory Domain"
            inv[Inventory Management]
            stk[Stock Tracking]
            inv --- MongoDB
            stk --- MongoDB
        end
        
        subgraph "Payment Domain"
            pay[Payment Processing]
            tx[Transaction Ledger]
            pay --- PostgreSQL
            tx --- PostgreSQL
        end
        
        subgraph "Analytics Domain"
            metrics[Metrics Collection]
            reports[Reporting]
            metrics --- Cassandra
            reports --- Elasticsearch
        end
    end
    
    subgraph "Data Integration"
        ksql[KSQL]
        cdc[CDC Connectors]
        etl[ETL Processes]
        
        Kafka --> ksql
        cdc --> Kafka
        ksql --> etl
    end
    
    MongoDB --> cdc
    PostgreSQL --> cdc
    Cassandra --> cdc
    
    etl --> DataLake
```

## Appendix B: Data Consistency Patterns by Service Type

```mermaid
flowchart TB
    subgraph "Data Consistency Patterns"
        direction TB
        
        subgraph "Transactional Services"
            saga[Saga Pattern]
            tcc[Try-Confirm/Cancel Pattern]
            2pc[Two-Phase Commit]
        end
        
        subgraph "Event-Driven Services"
            es[Event Sourcing]
            cqrs[CQRS]
            outbox[Outbox Pattern]
        end
        
        subgraph "Caching Patterns"
            cache[Cache-Aside]
            writeBehind[Write-Behind]
            readThrough[Read-Through]
        end
    end
    
    subgraph "Service Examples"
        checkout[Checkout Service]
        inventory[Inventory Service]
        catalog[Product Catalog]
        recommendations[Recommendations]
    end
    
    checkout --> saga
    checkout --> 2pc
    inventory --> es
    inventory --> outbox
    catalog --> cqrs
    catalog --> cache
    recommendations --> readThrough
```

## Appendix C: Database Migration Strategy

```mermaid
sequenceDiagram
    participant Legacy as Legacy Database
    participant Middleware as Migration Middleware
    participant New as New Service Databases
    participant CDC as Change Data Capture
    participant Events as Event Bus
    
    Note over Legacy, Events: Phase 1: Shadow Reading
    
    Legacy->>Middleware: Original Data Access
    Middleware->>New: Shadow Writes
    
    Note over Legacy, Events: Phase 2: Dual Write
    
    Legacy->>Middleware: Original Data Access
    Middleware->>Legacy: Write to Legacy
    Middleware->>New: Write to New System
    
    Note over Legacy, Events: Phase 3: CDC Migration
    
    Legacy->>CDC: Capture Changes
    CDC->>Events: Publish Change Events
    Events->>New: Update New System
    
    Note over Legacy, Events: Phase 4: Reverse Flow
    
    New->>Middleware: Primary Data Access
    Middleware->>Legacy: Synchronize for Legacy Systems
    
    Note over Legacy, Events: Phase 5: Legacy Retirement
    
    New->>New: Fully Migrated Operations
``` 